A standard assembly machine, for instance serving to put together small- to medium-sized articles such as hinges or lamp sockets, sits adjacent a conveyor or transfer machine that brings to it a succession of partially constructed and/or assembled workpieces that the assembly machine must perform some operation on. Normally the assembly machine is provided with one or more grippers and/or with a tool that performs some desired operation on the workpiece, often adding to the workpiece a part that is fed in by another conveyor.
Invariably such an assembler has at least one actuator that can move in a straight line. Normally two are provided, one to move in a holder and get a grip on the workpiece and another that carries a tool that operates on the workpiece and/or a part to be added to the workpiece. In the standard systems described in German patent 4,320,501 of G. Dornieden each workpiece is carried on a respective holder that is moved along by a special-duty conveyor through a plurality of work stations to some of which are delivered parts that are added to the workpiece. Immediately downstream of the furthest downstream work station is an emptying station that either strips each workpiece off its holder so that the holder can be recirculated to a point upstream of the furthest upstream work station and fitted with a new unfinished workpiece or that actually moves each holder and its workpiece along to another machine or set of machines. The conveyor is a worm extending the full length of the production line from the furthest upstream station to the furthest downstream station and formed with a continuous screwthread having pitched sections extending helically of the worm's axis alternating with unpitched sections lying in respective planes perpendicular to this axis. Each holder has formations complementarily engaged with this screwthread so that as the worm rotates the holders are advanced in steps, moving axially downstream when their formations engage the pitched screwthread sections and stationary while engaging the unpitched sections.
In German patent 4,328,988 of G. Dornieden and C. Tischendorf another system is described where the workpieces are moved, with or without holders, in a horizontal orbit about a vertical axis. To this end they are held in respective seats spaced angularly apart around a ring on the edge of a circular transfer plate that itself is driven step-wise. The drive for the transfer plate is mechanically synchronously coupled to a shaft carrying a stack of different cams each associated with a respective push-pull-actuator, e.g. a bowden cable, whose other end is coupled to a slide, gripper, or other device at a work station so that everything is done synchronously.
Normally as mentioned the movable elements, termed tools hereinafter, at the work stations that grip, machine, install parts on, and otherwise operate on the workpiece are moved between a working position in the station to a retracted position out of the way of the workpieces as they are stepped from station to station. In the rest position the tools are typically biased against the workpiece by a spring that, for instance, presses a rotating drill bit or screw into the workpiece, pushes a press-fit part into a seat on the workpiece, or somehow deforms the workpiece. The force of the spring must be set before each production run in accordance with the operation performed and the type of workpiece. Since the typical spring characteristic, that is spring force plotted against compression or extension, is not a straight line, it is not only necessary to reset the precompression of the spring during each changeover, but also often to replace the spring altogether with another having a different characteristic when the existing spring cannot be set to the needed force. Obviously this operation substantially increases the down time during changeover from one production run to another.